A solar cell is an essential device of a photovoltaic system that directly converts sunlight into electricity.
Generally, a solar cell is divided into a single-crystal silicon solar cell, a polycrystalline silicon solar cell, and a thin-film solar cell.
The single-crystal silicon solar cell has higher conversion efficiency than other kinds of solar cells and is suited for mass production through process improvement.
The polycrystalline silicon solar cell employs a low-grade silicon wafer as a source material. Although the polycrystalline silicon solar cell requires low fabrication costs, the polycrystalline silicon solar cell has lower conversion efficiency than that of the single-crystal silicon solar cell.
Since such single-crystal and polycrystalline silicon solar cells are prepared using a source material in a bulk state, these solar cells have problems of high material costs and a complicated fabrication process, thereby providing constraints on cost reduction.
To solve these problems, thin-film solar cells prepared through significant reduction in thickness of a substrate or through deposition of a thin-film solar cell on an inexpensive substrate such as a glass sheet have attracted attention in the art. Although the thin-film solar cell has lower conversion efficiency than the single-crystal and polycrystalline silicon solar cells, the thin-film solar cell has a possibility of reducing fabrication costs.
Recently, studies have been conducted to develop a thin-film solar cell using a compound semiconductor material such as CdTe, CuInSe2 (CIS), or CuInGaSe2 (CIGS), which has relatively high conversion efficiency. Particularly, many attempts have been made to further improve conversion efficiency of a thin-film solar cell using a CIS or CIGS compound semiconductor having relatively high conversion efficiency in a light-absorption layer.
Although the CIS or CIGS light-absorption layer is prepared by vacuum deposition, there are problems of complicated processing conditions, difficulty fabricating a large-area product, and large loss of a source material.
To solve these problems with vacuum deposition, a method for fabricating a non-vacuum CIS or CIGS light-absorption layer, which does not use vacuum apparatus, is known in the art. Particularly, a printing method for fabricating a CIS or CIGS thin-film is known as the most available method in terms of processing speed, processing cost, and formation of large-area products.
Fabrication of the CIS or CIGS light-absorption layer by the printing method generally includes a method using ink or paste composed of a precursor and a method using ink or paste prepared by forming CIG or CIGS nanoparticles and distributing the particles.
As an exemplary method using a precursor, a method by Mitzi et al., disclosed in Advanced Materials, 2008, 20, 3657-3662, includes dissolving a binary compound, such as Cu2S, In2Se3, or Ga2Se, in hydrazine to form a precursor ink, depositing the precursor ink on a conductive substrate, and heat-treating the precursor ink under a nitrogen atmosphere, thereby fabricating a CIGS light-absorption layer. Further, a method by Min et al., disclosed in Journal of Crystal Growth, 2009, 311, 2621-2625, includes preparing paste by dissolving Cu(NO3)2, In(NO3)3, Ga(NO3)3, and SeCl4 in an alcohol solvent and mixing an organic binder or the like therewith, depositing the paste on a conductive substrate, and then heat-treating the paste under a H2/Ar atmosphere, thereby fabricating a CIGS thin-film.
As an exemplary method using nano-particles, a method by Kapur et al., disclosed in Thin Solid Films, 2003, 431-432, and 53-57, includes synthesizing and distributing CuInGa oxide nano-particles, depositing the nano-particles on a conductive substrate, and heat-treating the nano-particles under an H2Se-gas atmosphere, thereby fabricating a CIGS light-absorption layer.
Thereamong, the method using a precursor has a problem in that a large amount of carbon impurities remains when heat treatment is performed under a hydrogen or nitrogen atmosphere. Further, when a solvent such as hydrazine is used, remaining carbon impurities can be decreased. In this case, however, there is a drawback in that industrial applicability is restricted due to highly explosive properties of hydrazine.
The remaining carbon impurities act as a main element causing reduction in conversion efficiency of a solar cell. Thus, in order to solve the problems of the method for fabricating a light absorption layer by printing, there is a need for a preparation method capable of minimizing remaining carbon impurities while using a stable organic solvent.